frequency-dependent selection

Examples of frequency-dependent selection in the following topics:

• Frequency-Dependent Selection

  • In frequency-dependent selection, phenotypes that are either common or rare are favored through natural selection.
  • Another type of selection, called frequency-dependent selection, favors phenotypes that are either common (positive frequency-dependent selection) or rare (negative frequency-dependent selection).
  • Negative frequency-dependent selection serves to increase the population's genetic variance by selecting for rare phenotypes, whereas positive frequency-dependent selection usually decreases genetic variance by selecting for common phenotypes.
  • Frequency-dependent selection allows for both common and rare phenotypes of the population to appear in a frequency-aided cycle.
  • Positive frequency-dependent selection reinforces the common phenotype because predators avoid the distinct coloration.

• Natural Selection and Adaptive Evolution

  • Natural selection drives adaptive evolution by selecting for and increasing the occurrence of beneficial traits in a population.
  • Natural selection only acts on the population's heritable traits: selecting for beneficial alleles and, thus, increasing their frequency in the population, while selecting against deleterious alleles and, thereby, decreasing their frequency.
  • Natural selection does not act on individual alleles, however, but on entire organisms.
  • As natural selection influences the allele frequencies in a population, individuals can either become more or less genetically similar and the phenotypes displayed can become more similar or more disparate.
  • Through natural selection, a population of finches evolved into three separate species by adapting to several difference selection pressures.

• Hardy-Weinberg Principle of Equilibrium

  • We can also write this as p + q = 1.If the frequency of the Y allele in the population is 0.6, then we know that the frequency of the y allele is 0.4.
  • From the Hardy-Weinberg principle and the known allele frequencies, we can also infer the frequencies of the genotypes.
  • The frequency of homozygous pp individuals is p2; the frequency of hereozygous pq individuals is 2pq; and the frequency of homozygous qq individuals is q2.
  • The frequency of heterozygous plants (2pq) is 2(0.6)(0.4) = 0.48.
  • The genetic variation of natural populations is constantly changing from genetic drift, mutation, migration, and natural and sexual selection.

• Nonrandom Mating and Environmental Variance

  • One reason is simple mate choice or sexual selection; for example, female peahens may prefer peacocks with bigger, brighter tails.
  • Traits that lead to more matings for an individual lead to more offspring and through natural selection, eventually lead to a higher frequency of that trait in the population.
  • For example, some turtles and other reptiles have temperature-dependent sex determination (TSD).
  • Alternatively, flowering plants tend to bloom at different times depending on where they are along the slope of a mountain, known as an altitudinal cline.
  • Explain how environmental variance and nonrandom mating can change gene frequencies in a population

• Population Genetics

  • Population genetics is the study of how selective forces change a population through changes in allele and genotypic frequencies.
  • The allele frequency (or gene frequency) is the rate at which a specific allele appears within a population.
  • Genetic drift and natural selection usually occur simultaneously in populations, but the cause of the frequency change is often impossible to determine.
  • Natural selection also affects allele frequency.
  • Together, the forces of natural selection, genetic drift, and founder effect can lead to significant changes in the gene pool of a population.

• No Perfect Organism

  • Natural selection cannot create novel, perfect species because it only selects on existing variations in a population.
  • Natural selection can only select on existing variation in the population; it cannot create anything from scratch.
  • When a neutral allele is linked to beneficial allele, consequently meaning that it has a selective advantage, the allele frequency can increase in the population through genetic hitchhiking (also called genetic draft).
  • One morph may confer a higher fitness than another, but may not increase in frequency because the intermediate morph is detrimental.
  • The dark-colored mice may be more fit than the light-colored mice, and according to the principles of natural selection the frequency of light-colored mice is expected to decrease over time.

• Stabilizing, Directional, and Diversifying Selection

  • If natural selection favors an average phenotype by selecting against extreme variation, the population will undergo stabilizing selection.
  • Over time, the frequency of the melanic form of the moth increased because their darker coloration provided camouflage against the sooty tree; they had a higher survival rate in habitats affected by air pollution.
  • Sometimes natural selection can select for two or more distinct phenotypes that each have their advantages.
  • Directional selection occurs when a single phenotype is favored, causing the allele frequency to continuously shift in one direction.
  • Different types of natural selection can impact the distribution of phenotypes within a population.In (a) stabilizing selection, an average phenotype is favored.In (b) directional selection, a change in the environment shifts the spectrum of phenotypes observed.In (c) diversifying selection, two or more extreme phenotypes are selected for, while the average phenotype is selected against.

• Genetic Drift

  • Genetic drift is the converse of natural selection.
  • Over successive generation, these selection pressures can change the gene pool and the traits within the population.
  • Over time, the selection pressure will cause the allele frequencies in the gorilla population to shift toward large, strong males.
  • Unlike natural selection, genetic drift describes the effect of chance on populations in the absence of positive or negative selection pressure.
  • The Hardy–Weinberg principle states that within sufficiently large populations, the allele frequencies remain constant from one generation to the next unless the equilibrium is disturbed by migration, genetic mutation, or selection.

• Allopatric Speciation

  • Thus, the frequency of an allele at one end of a distribution will be similar to the frequency of the allele at the other end.
  • Thus, their allele frequencies at numerous genetic loci gradually become more and more different as new alleles independently arise by mutation in each population.
  • Typically, environmental conditions, such as climate, resources, predators, and competitors for the two populations will differ causing natural selection to favor divergent adaptations in each group.
  • The nature of the geographic separation necessary to isolate populations depends entirely on the biology of the organism and its potential for dispersal.
  • In Hawaiian honeycreepers, the response to natural selection based on specific food sources in each new habitat led to the evolution of a different beak suited to the specific food source.

• Defining Population Evolution

  • A single individual cannot evolve alone; evolution is the process of changing the gene frequencies within a gene pool.
  • Just as mutations cause new traits in a population, natural selection acts on the frequency of those traits.
  • When selective forces are absent or relatively weak, gene frequencies tend to "drift" due to random events.
  • Therefore, the frequency of a gene may increase in a population through genetic hitchhiking if its proximal genes confer a benefit.
  • As mutations create variation, natural selection affects the frequency of that trait in a population.